1. Field of the Invention
This invention relates to a semiconductor device and a method of producing the same. More particularly, this invention relates to improvements in a semiconductor device of SOI (silicon on insulator) structure using polycrystalline silicon and a method of producing such a semiconductor device.
2. Description of the Prior Art
A semiconductor device of SOI structure using polycrystalline silicon is now available, and such a structure is considered as an effective means for realizing desired three-dimensional integration of semiconductor devices. That is, three-dimensional integration of a semiconductor device can be attained by thermal oxidation of a silicon wafer to deposit polycrystalline silicon on an insulator layer which is a film of SiO.sub.2 and then heat treating this polycrystalline silicon as by laser annealing to promote crystallization of the polycrystalline silicon, thereby forming a semiconductor device such as a MOS transistor on the polycrystalline silicon layer deposited on the SiO.sub.2 film.
However, in a semiconductor device using polycrystalline silicon as described above, the polycrystalline silicon layer includes portions which remain in the form of the polycrystal, and the presence of many crystal grain boundaries leads to a disadvantage such as degradation of electrical properties including carrier mobility. This disadvantage can be obviated by decreasing the number of grain boundaries which provide the source of the disadvantage, and the number of grain boundaries can be decreased by increasing the size of the individual crystal grains. However, in the case of the polycrystalline silicon layer which is not doped with an impurity, application of simple heat treatment hardly causes crystal growth, and, for this reason, the size of the crystal grains cannot be increased. On the other hand, the size of the crystal grains can be increased when the polycrystalline silicon layer is doped with an impurity. In this case, in order to increase the size of the crystal grains, it is necessary to dope the polycrystalline silicon layer with an impurity having a high concentration. Therefore, even when the number of grain boundaries in the polycrystalline silicon layer can be decreased by such a method, it is difficult to utilize this polycrystalline silicon layer as an active region (a current path region) of the semiconductor device because the impurity concentration of the polycrystalline silicon layer is high.
Further, in a film of polycrystalline silicon commonly used in a semiconductor device and having a thickness smaller than 5,000 .ANG., the size of crystal grains is small and, therefore, there are many crystal grain boundaries. Also, because the individual crystals have their crystallographic axes oriented at random, the performance of the semiconductor device cannot be sufficiently improved.